1. Field of the Invention
The present invention generally relates to a chemically amplified resist composition, and more particularly, the present invention relates to a resist composition comprising a photosensitive polymer having lactone in its backbone.
2. Description of the Related Art
As the integration density and complexity of semiconductor devices continue to increase, the ability to form ultra-fine patterns becomes more and more critical. For example, in 1-Gigabit or higher semiconductor devices, a pattern size having a design rule of 0.2 μm or less is needed. For this reason, in lithography processes, the lower wavelength ArF eximer laser (193 nm) has emerged as a preferred exposure light source to the more conventional and higher wavelength KrF eximer laser (248 nm).
However, compared with conventional (KrF) resist materials, resist materials which are suitable for use with the ArF eximer laser suffer from a variety drawbacks. The most serious problems relate transmittance and resistance to dry etching.
Almost all well-known ArF resist compositions contain (meth)acryl-based polymers. Among these polymers, a methacrylate copolymer having an alicyclic protecting group, which is expressed by the formula below, has been suggested (J. Photopolym. Sci. Technol., 9(3), pp. 509 (1996))

This polymer has an adamantyl group, which contributes to enhancing resistance to dry etching, and a lactone group, which improves adhesiveness, in its methacrylate backbone. As a result, the resolution of the resist and the depth of focus are improved. However, resistance to dry etching is still weak, and serious line edge roughness is observed after line patterns are formed from the resist layer.
Another drawback of the aforementioned polymer is that the raw material used to synthesize the polymer is expensive. In particular, the manufacturing cost of a polymer having a lactone group, which is introduced to improve adhesiveness, is so high that its practical use as a resist is difficult.
As another conventional resist composition, a cycloolefin-maleic anhydride (COMA) alternating polymer having the following formula has been suggested (J. Photopolym. Sci. Technol., Vol. 12(4), pp. 553 (1999), and U.S. Pat. No. 5,843,624)

In the production of a copolymer, such as a COMA alternating polymer having the formula above, the production cost of raw material is relatively inexpensive, but the yield of the polymer sharply decreases. In addition, the transmittance of the polymer is very low at a short wavelength region, for example at 193 nm. The synthetic polymers have in their backbone the alicyclic group, which exhibits prominent hydrophobicity, and thus the adhesiveness to neighboring material layers is very poor.
The copolymer has a glass transition temperature of 200° C. or more due to the structural characteristic of the backbone. As a result, it is difficult to carry out an annealing process for eliminating free volume from the resist layer formed of the polymer, and accordingly the resist layer is influenced by ambient conditions which can cause, for example, a T-top profile of corresponding resist patterns. In addition, the resist layer itself becomes less resistant to ambient conditions during post-exposure delay, so that many problems can occur during subsequent processes with respect to the photoresist layer.
To improve the resolution of the resist layer, the polymer system must be charged with a polar group. In recent years, a technique of introducing a lactone group into a methacrylate monomer having an alicyclic protecting group, using the following alicyclic compounds with a lactone group, has been suggested so as to enhance the resistance to dry etching (J. Photopolym. Sci. Technol., Vol. 13(4), pp. 601 (2000), and Japanese Patent Laid-open No. hei 12-26446):

Unfortunately, the yield of the monomer having the above formula is so low as to substantially increase manufacturing costs.